
OVERVIEW
Students will (1) measure the density of water, (2)
measure and compare the density of salt water, (3) demonstrate changes in
density by adding marbles to a floating plastic container until it sinks,
and (4) compare their result with calculated predictions.
CONCEPTS
 A useful definition of a gram is the mass of one
milliliter of pure water.
 Whether an object will float depends on the amount of water that object
displaces.
MATERIALS
 Clear plastic containers about 15 cm (6 in) square and 4 cm (1.6 in)
deep (for example those used for takehome food such as salads). One per
group.
 Graduated cylinders, 50 ml or 100 ml
 Scales, 0 to 200 grams
 Glass marbles, 125 per group (pennies will also work and are less likely
to roll around)
 Glass or plastic bowls, large enough to hold one of the clear plastic
containers listed above
 Table salt, about 4 gm per group
 Paper towels
PREPARATION
Clear plastic containers made with thin plastic will give the best results
because the thin plastic will have little effect on volumes and densities.
If you cannot find these, however, you can try similar containers such as
Tupperware, but results may not be as accurate. Separate lids and bottoms
of the plastic containers. Each group should have 1 lid or 1 bottom. Use the
graduated cylinders to pour water into the lid or bottom to find its liquid
capacity (volume). They should be in the 500 to 700 ml
range. These containers will be the boats used in Part II. Measure the mass
of 10 or 20 marbles together, and then calculate the mass per marble. Students
will need this value for Part II of the activity. Alternatively, before Part
II each group of students can weigh 10 or 20 marbles, and calculate the mass
per marble themselves, rather than having the teacher provide them with this
number.
PROCEDURE
Engagement
One of the most important molecules on Earth is water. Water is commonly
used as a reference for physical properties. One such physical property, density,
is defined as the measure of a materialšs mass (e.g., in grams) divided by
its volume (e.g., in milliliters) (d=m/v). The density of water, 1 g/ml, is
also used as a means of comparison called specific gravity.
Water is defined to have a specific gravity of 1 (no units). Objects with
a specific gravity of less than one will float, while objects with a specific
gravity of more than one will sink. Seawater has an average specific gravity
of 1.028 with 3.5 g of dissolved salts for every 100 g of pure water. Ship
designs and carrying capacity are based upon the known density of water. The
human body is about 70% water and has about the same average density as water.
Activity
Part I
Determining the density of tap water:
 Measure the mass of the empty graduated cylinder. Record the weight.
 Fill the cylinder with water to the 100 ml line. This is the volume.
 Measure the mass of the cylinder with water.
 Subtract the mass of the empty cylinder from the mass of the filled cylinder.
 Divide the mass of the water by its volume. This will yield the density
of the tap water. Record your result.
Determining the density of tap water with salt:
 Use an eyedropper to remove 2 g (2 ml) of water from the cylinder.
 While the cylinder is on the scales, add 2 g of salt.
 Read the new water level inside the cylinder. This is the new volume.
 Divide the mass of the water inside the cylinder by its new volume. This
is the density of the salt water. Record your result.
 Compare the densities of the salt water and the fresh water.
Part II
 Measure the volume of the plastic container (boat). Fill a graduated
cylinder with 100 ml of water and pour it into the hull of your boat. Do
this as many times as necessary until the boat is full. Be sure to keep
track of how many times you refilled the cylinder. On the last cylinder
of water, any water left over in the graduated cylinder must be subtracted
from the 100 ml originally in the cylinder. Multiply the number of times
you refilled your cylinder by 100, then subtract the amount of water left
over in the last cylinder. This is the total volume, TOTAL(ml). Record your
answer.
 Find the mass your boat will carry. Since one milliliter of water is equal
to one gram, the volume in ml of your boat also equals the mass it can carry
in grams. Write your total mass, TOTAL(g).
 Calculate the number of marbles your boat will hold. Divide your TOTAL(g)
by the mass of the marble (from ŗPreparation˛ section). This equals the
number of marbles your boat should be able to carry. Record this number.
Calculate 90% of that number by multiplying by 0.9.
 Count out 90% of the calculated number of marbles and place them into
your boat. Be sure the marbles are distributed evenly to avoid tipping of
the boat.
 Carefully place the boat, with the number of marbles calculated in step
4 inside the boat, into the bowl of water.
 Add more marbles to your boat, one at a time, counting and adding these
to the previous number of marbles. Continue this until the boat sinks. Remember
to place the marbles carefully to maintain a level boat. Record the number
of marbles it took to sink the boat.
 Compare the calculated number of marbles to the actual number of marbles
held afloat by your boat before it sank. If the numbers are different, what
factors may have contributed to that difference?
 To repeat the experiment, be sure to first dry the marbles and the inside
of your boat.
 Optional: Add a significant amount (e.g., 20 grams or more) of salt to
the water, then repeat the experiment. Do you find a difference? Why?
Explanation
Part I: A useful definition of a gram is the mass of one cubic centimeter
(cm 3 ), also called a milliliter (ml), of pure water. The density of pure
water varies with temperature: water contracts until almost freezing and expands
into a gas when boiling. The density of pure water is 1 g/ml at 4°C (39°F);
however this changes by less than 0.2% at room temperature. Adding salt increases
the density of the water.
Part II: For any floating object, the buoyant force
equals the weight of the liquid displaced (Archimedešs Principle). A plastic
boat which holds 500 ml of water will support 500 g of any denser material.
A less dense load of the same mass will have a higher center of gravity and
will cause the boat to tip.
EXTENSION
Discuss whether it will be easier for a person to float in salt water
or fresh water. Why? Have any of the students noticed this difference?
For stability, the center of gravity of a boat must be below the center
of buoyancy as in Figure 1 (right). The boat in
Figure 1 (left) will tip over. Standing up in a
canoe shifts the center of gravity and can cause it to flip over. What
other types of boats are designed to be more stable than canoes? What
are the advantages of the canoe design over more stable boats?
VOCABULARY
 buoyant (buoyancy): 1) the tendency of an object to float or rise
when submerged in a fluid. 2) the power of a fluid to exert an upward force
on a body placed in it.
 density: mass per unit volume of a substance. Usually expressed
as grams per cubic centimeter. For ocean water with a salinity of 35 at
0C, the density is 1.028 g/cm 3 .
 gram: 1/1000 of a kilogram. Abbreviated g or gm.
 specific gravity: the ratio of density of a given substance to
that of pure water at 4ēC and at a pressure of one atmosphere.
 volume: the amount of space occupied by a threedimensional object.
SOURCE: San Juan Institute
Activity Series.
STANDARDS & BENCHMARKS
 Science Standard 1, Grades 68 Knows the properties that
make water an essential component of Earth system (e.g., its ability to
act as a solvent, its ability to remain a liquid at most Earth temperatures)
 Science Standard 1, Grades 35 Knows that water can change
from one state to another (solid, liquid, gas) through various processes
(e.g., freezing, condensation, precipitation, evaporation)
 Science Standard 10, Grades K2 Knows that different objects
are made up of many different types of materials (e.g., cloth, paper, wood,
metal) and have many different observable properties (e.g., color, size,
shape, weight)
 Science Standard 10, Grades K2 Knows that things can
be done to materials to change some of their properties (e.g., heating,
freezing, mixing, cutting, dissolving, bending), but not all materials respond
the same way to what is done to them
 Science Standard 10, Grades 35 Knows that objects can
be classified according to their properties (e.g., magnetism, conductivity,
density, solubility)
 Science Standard 10, Grades 35 Knows that properties
such as length, weight, temperature, and volume can be measured using appropriate
tools (e.g., rulers, balances, thermometers, graduated cylinders)
 Science Standard 10, Grades 35 Knows that materials have
different states (solid, liquid, gas), and some common materials such as
water can be changed from one state to another by heating or cooling
 Science Standard 10, Grades 68 Knows that atoms often
combine to form a molecule (or crystal), the smallest particle of a substance
that retains its properties
 Science Standard 10, Grades 68 Knows that atoms are in
constant, random motion (atoms in solids are close together and don't move
about easily; atoms in liquids are close together and stick to each other,
but move about easily; atoms in gas are quite far apart and move about freely)
 Math Standard 4, Grades 35 Understands the basic measures
perimeter, area, volume, capacity, mass, angle, and circumference Math Standard
4, Grades 35 Knows approximate size of basic standard units (e.g., centimeters,
feet, grams) and relationships between them (e.g., between inches and feet)
 Math Standard 4, Grades 35 Understands relationships
between measures (e.g., between length, perimeter, and area)
 Math Standard 3, Grades 35 Adds, subtracts, multiplies,
and divides whole numbers and decimals
 Math Standard 3, Grades 68 Adds, subtracts, multiplies,
and divides whole numbers, fractions, decimals, integers, and rational numbers
 Math Standard 3, Grades K2 Adds and subtracts whole numbers
 Math Standard 3, Grades 68 Uses proportional reasoning
to solve mathematical and realworld problems (e.g., involving equivalent
fractions, equal ratios, constant rate of change, proportions, percents)
